1. Field of the Invention
This invention relates generally to bicycles and more particularly to bicycle drive mechanisms.
2. Description of the Prior Art
A standard bicycle has a drive mechanism including a crank-and-pedal assembly for rotating a front gear assembly, a rear gear assembly attached to the rear wheel, and a chain coupling the front gear assembly to the rear gear assembly. A mechanism is usually provided to change the gearing ratio of the bicycle.
One alternative to the chain-and sprocket drive mechanism described above is a drive shaft type mechanism including a front gear assembly coupled to a crank-and-pedal assembly, a rear gear assembly coupled to the rear wheel, and a rotary drive shaft coupling the front gear to the rear gear.
Various types of bicycle drive shaft mechanisms are described in U.S. Pat. Nos. 649,878 of Scharbach, 479,470 of Smallwood, 2,378,634 of Hussey, 624,964 of Oberhammer, 3,861,715 of Mendoza, 675,067 of Nichols, 662,891 of Copeland, and 2,475,654 of Watson. Scharbach's patent, which is typical of the above cited patents, describes a bicycle having a drive shaft R coupled at one end to a crank C and coupled at its outer end to a cog gearing ring H which engages a disk gear G attached to the rear axle. The cog gearing ring H is slidably attached to drive shaft R and is capable of axial movement along a portion of the drive shaft under the manual control of a rod E.sup.2. Scharbach's gears are changed by adjusting rod E.sup.2 to cause cog gearing ring H to move to one of cog gears H.sup.1, H.sup.2, or H.sup.3.
A disadvantage of prior art bicycle drive shaft mechanisms is that the number of gearing ratios is limited. For example, in Scharbach's only three discrete gearing ratios are available, corresponding to cog gears H.sup.1, H.sup.2, and H.sup.3. Due to engineering limitations of this type of disk gear, it is unlikely that very many more cog gears could be added to extend the gearing range.
Another disadvantage of the prior art is that the gearing ratios have to be changed manually. Besides being inconvenient for a bicycle rider to have to manually shift gears all the time, a manual gear shift often can not be used when the gears in mesh are under a heavy load, i.e. such as when a bicyclist is riding up a steep hill.